Stereoscopic vision is possible because our eyes are displaced horizontally. As a consequence, for any given position of the eyes, the variation due to scene geometry causes a much larger range of horizontal disparities than vertical disparities. Yet until recently, there was little evidence that this anisotropy was reflected in the brain.

Now Cumming (2002: Nature 418:633) has shown that cells in V1 respond to a wider range of horizontal than vertical disparities, irrespective of their orientation preference. This presents a challenge to existing models of disparity-sensitive cells, which predict that the sensitivity to vertical/horizontal disparities should depend on the orientation tuning (eg, cells tuned to vertical orientations should respond to a wider range of vertical disparities than horizontal, which is not observed). We present some ways in which existing models can be extended to account for these observations. One possibility is that a given complex cell receives input from many binocular subunits with different horizontal position disparities but similar vertical position disparities, so that the envelope of the sum is horizontally elongated. Another is that endstopping in monocular subunits suppresses the response to disparities with a significant vertical component. New experiments will be needed to reveal if these suggestions are correct.